The present disclosure is generally directed to semiconductors of the formulas/structures as illustrated herein and processes of preparation and uses thereof. More specifically, the present disclosure in embodiments is directed to TFT polymers of the formulas as illustrated herein and more specifically to a number of stabilized polythiophenes wherein there is introduced into the polythiophene atoms of higher electronegativity, such as O (3.44, Pauling's electronegativity), and N (3.04) than C (2.55) at the beta and/or gamma positions of the 3-substituent of the thiophene unit, and which resulting polythiophenes are more stable than poly(3-alkylthiophene)s, such as poly(3-hexylthiophene) (P3HT), and which disclosed polythiophenes can be selected as semiconductors for polymer thin film transistors, and also which can be selected as solution processable and substantially stable channel semiconductors in organic electronic devices, such as thin film transistors, and which transistors are stable in air, that is do not substantially degrade over a period of time when exposed to oxygen. Although not desiring to be limited by theory, it is believed that the inductive effect and insignificant resonance (or mesomeric) effect of atoms like O and N, and that such atoms at the beta and/or gamma positions would have an electron withdrawing characteristic on the polymer backbone that there is lowered the HOMO (highest occupied molecular orbital) energy level of the polythiophene polymer thus resulting in an increased stability against oxygen and in enhancing transistor performance, such as field effect mobility. More specifically, it is believed that although O and N atoms have a larger electronegativity than carbon (C), the net polar effect of the substituents containing O or N, such as OH, OR, is electron release (donating) when they are attached directly to the aromatic moiety of the thiophene primarily because of their electron releasing resonance effect, thus poly(3-alkoxythiophene) is usually more sensitive to photoinduced oxygen doping than poly(3-alkylthiophene). To eliminate or reduce the resonance effect of O and N atoms with the present disclosure, there is inserted, for example, one or two methylene groups between the O or the N and the thiophene moiety since the O and N would not involve the conjugation of the thiophene ring. However, the electron withdrawing inductive effect to the thiophene ring would be still maintained to some extent. The net polar effect of these substituents would be less electron-releasing than alkyl substituents, and thus lower the HOMO energy level of the polymer compared with poly(3-alkylthiophene), therefore an increase in the stability of these polymers towards oxidative doping by oxygen results.
There are desired electronic devices, such as thin film transistors, TFTs, fabricated with a semiconductor of the formulas as illustrated herein, and which semiconductors possess excellent solvent solubility, and which can be solution processable; and which devices possess mechanical durability and structural flexibility, characteristics which are desirable for fabricating flexible TFTs on a number of substrates, such as plastic substrates. Flexible TFTs enable the design of electronic devices with structural flexibility and mechanical durability characteristics. The use of plastic substrates together with the semiconductor of the formulas as illustrated herein can transform the traditionally rigid silicon TFT into a mechanically more durable and structurally flexible TFT design. This can be of particular value to large area devices such as large area image sensors, electronic paper and other display media. Also, the selection of the semiconductors of the formulas as illustrated herein possess in embodiments extended conjugation for integrated circuit logic elements for low end microelectronics, such as smart cards, radio frequency identification (RFID) tags, and memory/storage devices, and enhance their mechanical durability, and thus their useful life span.
A number of semiconductor materials are not, it is believed, stable when exposed to air as they become oxidatively doped by ambient oxygen resulting in increased conductivity. The result is large off current, and thus a low current on/off ratio for the devices fabricated from these materials. Accordingly, with many of these materials, rigorous precautions are usually undertaken during materials processing and device fabrication to exclude environmental oxygen to avoid or minimize oxidative doping. These precautionary measures increase the cost of manufacturing therefore offsetting the appeal of certain semiconductor TFTs as an economical alternative to amorphous silicon technology, particularly for large area devices. These and other disadvantages are avoided or minimized in embodiments of the present disclosure.